Three-dimensional diagnostic images of the brain, spinal cord, and other body portions are produced by diagnostic imaging equipment such as CT scanners, magnetic resonance imagers, and the like. These imaging modalities often provide structural detail with a resolution of a millimeter or better.
Image guided surgery systems have been developed to utilize this data to assist the surgeon in presurgical planning and in accurately locating a region of interest within the body of a patient. In the operating arena, the image guided surgery systems are used to display position and orientation of a surgical tool in its correct location with respect to the images of the patient. Surgical tools typically include a trackable handle portion and a tool head which may be inserted into the patient's body. One example of an image guided surgery system is U.S. Pat. No. 5,517,990, Stereotaxy Wand and Tool Guide, to Kalfas et al. issued May 21, 1996, incorporated by reference herein.
Three and sometimes four views of image data are displayed on a monitor visible to the surgeon. These views typically include axial, sagittal, and coronal views of the patient. A fourth oblique view is sometimes displayed, presenting image data in a plane orthogonal to a tip of the tool. The location of the tip of the tool, the tool's trajectory, and diameter of the tool head are displayed on one or more of these images. The algebraic distance between the tip of the tool and a desired position may also be displayed numerically on the monitor.
Given the nature of image guided surgery procedures, it is necessary to be able to track the location of the tip of the tool, the tool's trajectory, and diameter of the tool head with a high degree of precision, often requiring calibration to less than a millimeter in accuracy. The tools are tracked in an operating room or other area by use of a tracking system or localizer. The tracking system tracks the tools by virtue of three or more spaced apart position signaling devices, such as infrared emitters or reflectors, connected to the tool in a fixed relation thereto. The position signaling devices are positioned in a unique pattern for each tool in order to allow the tracking system to be able to distinguish one tool from another. In other words, the unique pattern can be said to characterize the tool.
A central computer coupled to the tracking system is preprogrammed with information related to where the tip and trajectory of each tool is with respect to the tool's position signaling devices and with information related to the diameter of the tool head. For instance, with respect to a tracked probe having three infrared emitters, the central computer maintains information related to where the tip of the probe is with relationship to a selected point on a plane defined by the three infrared emitters. Based on this information, a precise location of the tip can be calculated by the central computer and displayed on one of the monitors.
In a variety of surgical tools such as drills, probes, endoscopes, etc. it is often beneficial to a surgeon or other individual to make changes to the tool which may affect the positioning of the tip as well as the diameter of the tool head. For instance, on a surgical drill it is often helpful for the surgeon to be able to change the size and length of a drill bit situated in the tool to accommodate different surgical procedures. Further, with respect to the probes, it is often desirous to replace different length and diameter shafts on the probe handle in order to reach different regions in the patient.
Unfortunately, because the position of the tip of each tool with respect to the tool's position signaling devices are preprogrammed into a memory associated with each tool and passed along the central computer along with information on the diameter of the tool head, changes to the tool which affect the location of the tip and diameter of the tool head cannot easily be made. If changes are made, an operator needs to determine the new relationship between the tip of the tool and the tool's position signaling devices and enter this information into the central computer. Further, information related to a new diameter of the tool head may also need to be entered. This process is time consuming and cumbersome. If the new information is not entered into the central computer, the tip of the tool will not be properly tracked and displayed on the monitor.
The present invention provides a new and improved method and apparatus for calibrating a surgical tool which addresses the above-referenced matters, and others.